Can you explain how ADHD and things being hard in the way you describe are connected? I don't see the connection, but if it's the case, I should go see a doctor...
At a very high level, the answer is Dopamine. Normal peoples brains have a ready supply of it available and their brains generate some in preparation for doing a task and then continue to generate more as they keep working on things, this is the task positive reward loop keeping the brain focused. For people with adhd, this entire feedback loop is more or less fucked. You don’t get dopamine as you start a task, you don’t get it when you persist with a task, and you don’t get it when you complete a task. So getting things done basically provides no reward, as a result all you get is the negatives from doing the thing, it tires you out and depletes the dopamine reserves you do have available.